Dislocation mediated lattice bending in 1,6-di (N-carbazolyl)-2,4 hexadiyne (DCHD) polydiacetylene droplets
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Droplets of 1,6-di (N-carbazolyl)-2,4 hexadiyne (DCHD) polydiacetylene were prepared by room temperature evaporation of dilute (0.01 wt. %) solution of the monomer in chloroform onto amorphous carbon-coated mica substrates. High Resolution Electron Microscopy (HREM) and Selected Area Electron Diffraction (SAED) revealed small crystallographically textured droplets (~1 /mm diameter) with cracks parallel to the [001] chain direction. The droplet geometry allowed us to investigate the organization of the polymer near surfaces. It was found that the curvature of the droplet edge caused a local bending of the polymer crystal lattice. Direct imaging of the molecular structure near the droplet surface revealed that the mechanism of lattice bending was by the formation of edge dislocations. Dislocations were etched in some droplets to gain information about perturbations in structure and reactivity near the core.
I. INTRODUCTION The organization of polymers near defects is a general question of both scientific and technological interest. Defects in polymer systems have not been thoroughly studied because of the difficulties in imaging organic materials with high energy electron beams. Also, defects are difficult to isolate and study in detail because most polymer systems have small crystallites surrounded by regions of amorphous material. The optical and electromagnetic transport properties of polymers should be limited by the structure and density of defects that serve as sinks for photons and charge carriers. Mechanisms of deformation may also be affected by the presence of chain ends or other defects in the bulk of the crystal. The reorganization of diacetylene molecules near surfaces and defects in general is explored in this paper. 1,6-di (N-carbazolyl)-2,4 hexadiyne polydiacetylene (DCHD) was chosen for this experiment because it has been relatively well studied and exhibits electron radiation resistance that allows High Resolution Electron Microscopy (HREM) to be performed.1 The high degree of crystallinity also allows the effect of amorphous regions or small crystallites to be reduced. Droplet experiments using Transmission Electron Microscopy (TEM) have been shown by Henkee et al.2 and Martin et al? to be useful in determining the organization of polymers as they approach a constraining surface. Martin et al. discerned that poly(imide) molecules preferentially oriented in the plane of the substrate and weakly parallel to the edge of the droplet. As the thickness of the droplet increased, the orientation effect decreased. Droplets are useful in that near the edge the sample thickness decreases such that HREM imaging 3150 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 11, Nov 1992 Downloaded: 07 Apr 2015
becomes feasible, minimizing artifacts produced by other methods of sample preparation. Defects in crystalline diacetylenes have been previously studied by Young's group using TEM.1'4"7 Young and Petermann4 showed dislocations in poly(toluene sulfonate) polydiacetylene crystals with a line direction pe
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